Those involved in the industry seek an efficient, lightweight, and compact internal combustion engine having increased horsepower. To achieve this it is necessary to push engine design toward its mechanical limits. Increasing combustion pressures in the combustion chamber requires higher combustion temperatures, faster piston speeds and increased mechanical forces. As a result, the piston and associated components are placed under greater mechanical stress.
In order to perform satisfactorily and survive in such an environment it is necessary to provide a piston that has improved cooling capabilities, increased strength, and a short compression height to reduce its mass. It is also important that such a piston is easy to manufacture with a high level of quality.
It is known to provide a piston with a closed piston cooling gallery. An example of this is shown in U.S. Pat. No. 4,581,983 to Moebus. The closed piston cooling gallery of Moebus is provided by welding a top portion of the piston to a bottom portion of the piston along a planar surface. The top and bottom portions of the piston each have a portion of the cooling gallery disposed therein. This piston has an excessively tall compression height making it heavy and unsuitable for high-speed operation. This piston is also difficult to manufacture and does not have the strength to withstand the increased stresses of the higher combustion pressures. The closed piston cooling gallery as configured in Moebus does not provide a height sufficient to permit adequate shaking of cooling fluid within the closed piston cooling gallery. Therefore, the efficiency of cooling of the piston is inadequate.
A method of increasing the contact between oil, or another cooling fluid, and the interior of the piston is by increasing the surface area of the interior of the piston, thereby providing more area for the oil to contact and from which it can absorb heat. U.S. Pat. No. 2,523,699 issued to Holt discloses a series of ribs projecting inwardly from the interior wall of the piston skirt. These ribs increase the heat dissipating area of the piston that is in contact with the oil as the oil is shaken by the reciprocating motion of the piston. However, although the ribs increase the surface area of the piston that may be in contact with the oil, the speed and direction of the oil within the gallery are based on the speed and orientation of the piston. Additionally, the intricate piston design set forth in Holt is very difficult to produce via forging or machining processes. Therefore, the piston disclosed in Holt is practical for use solely with cast pistons, and casting may introduce impurities into the cast product. These impurities can decrease the density of the product and thus decrease the product's resistance to deformation at high temperatures and pressures.
U.S. Pat. No. 6,532,913 to Opris shows a piston having an annular cooling fin extending from an upper inner surface of the piston. However, the continuous annular fin acts in a manner similar to that of Holt, that is, to dissipate heat by increasing the surface area on the inner surface of the piston. Although the fin increases the surface area of the piston to facilitate the dissipation of heat, as well as increasing the surface area of the piston that may be in contact with the oil, the speed and direction of the oil within the gallery will still be based on the speed and orientation of the piston. Therefore, as the piston moves in an axial direction, the majority of the oil will also be moving in the axial direction. The cooling fin has little or no impact on the movement of the oil while the fluid is within the oil gallery.
The present invention is directed to overcoming one or more of the problems set forth above.